At the present time, many electronic circuits are integrated into small components or modules that are then mounted on larger carriers, such as printed circuit boards and the like. One of the major problems that arises in this process is the differences in thermal expansion between the material making up the integrated modules and the material making up the carriers. For example, a commonly employed material for integrated modules is ceramic. Ceramic integrated modules are generally mounted on some type of printed circuit board employing a hard plastic. The difference in thermal expansion in these materials produces a thermal expansion mismatch that can seriously effect the interconnect reliability and/or the life of the product.
Generally, the integrated modules have electrical traces distributed around the lower surface. In the prior art, the electrical traces of the integrated modules are electrically and physically connected to mounting and connecting pads on a printed circuit board by means of solder balls. The center of the module's lower surface can be considered as a zero stress or neutral point with connections spaced from the center having more stress, and the farther connections are spaced from the center the more stress they realize. Some of this stress is relieved by the solder balls, which provide a small amount of resiliency or movement. To further relieve this stress, the height of the solder ball is increased. The standoff height and the distance of the solder joint from the thermal expansion neutral point are well known to be the most critical factors responsible for solder joint thermal fatigue. Higher standoff heights and smaller distances from the thermal expansion neutral point are preferable.
Present demands for lower profile structures and larger integrated modules pose serious reliability issues. Generally, a customer specifies a maximum height (the distance between the surface of the mounting board and the upper surface of the module) that can be tolerated. The manufacturer must then arrange a compromise between the standoff height and the amount and position of the components in the module. Clearly, as the standoff height is increased to improve the interconnect thermal fatigue life, the number of components in the module must be reduced to achieve the specified maximum height.
Accordingly it would be highly desirable to provide apparatus and a method of reducing the standoff height and improving solder joint thermal fatigue life without reducing the reliability.